Only "dirty" fighting tactic I can think of is attacking a person from behind.
With regard to self defence I've been told you do what it takes to get out of the situation as quickly as possible - and you don't let it become a "fight" - fights you can lose. So you take the SoB out as fast as possible and get the Hell out of Dodge.
On the school bus I used to get the old knuckle on top of the head from behind - hurts like hell - and I'd tun around and none of the 4 who could reach me easily (seat immediately behind me and the one behind that) would admit to doing it.
Put up with it for months until I got so fucked off I rose and spun, lashing out my arm in a backfist into the first face I encountered. It was a perfectly scientific movement, roll of hips and shoulders combined with straightening of arm - all nicely fuelled by adrenaline because I was fucked off immensely - my fist made a full 180-degree swing before I hit anything.
Fist smacked into this blokes jaw with a satisfying crack, his head whips sideways and he's got this fucking-great welt on the side of his face, my hand flows through as I go from sitting facing forwards to standing facing rearwards.
Ended up standing looking down at him (a rare feeling for the school short-arse, but he was sitting down at the time) and he let out this indignant "I didn't do it!"
I snarled "You fuckin' let it happen. Pass it on." And sat back down, facing the front, quite prepared to get hit again and was in the mood to go ballistic.
That much adrenaline in my system I didn't give a fuck any more.
Y'know, no one ever hit me on the head from behind after that day.
Odd, eh. 
Spent the next dunno how long wondering why I hadn't done that the first time I was hit from behind - would've saved myself several unpleasant months...
Motorbike Camping for the win!
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